1. Field of the Invention
The present invention relates to a liquid display crystal display device, and more particularly, to a liquid crystal display device, which recognizes a variation in liquid crystal capacitance according to a touch to be capable of sensing whether or not the device is touched and the position of a touched region.
2. Discussion of the Related Art
As the information-oriented age has arrived, the field of displays visually expressing electric data signals has been rapidly grown, and in order to satisfy this growth, various flat display devices having characteristics, such as thin profile, light weight, and low power consumption, have been developed and rapidly replaced a conventional cathode ray tube (CRT).
Specifically, the flat display devices include a liquid crystal display device (LCD), a plasma display panel device (PDP), a field emission display device (FED), an electro luminescence display device (ELD), etc. All the above flat display devices essentially include a flat display panel to display an image. The flat display panel consists of a pair of transparent insulating substrates, which are bonded to each other under the condition that an intrinsic luminescent or polarizing material layer is interposed therebetween.
A liquid crystal display device displays an image by adjusting the light transmittance of liquid crystals using an electric field. Therefore, the liquid crystal display includes a display panel having liquid crystal cells, a backlight unit to irradiate light onto the display panel, and driving circuits to drive the liquid crystal cells.
The display panel is configured such that a plurality of gate lines and a plurality of data lines intersect each other to define a plurality of unit pixel regions. Here, in each of the pixel regions, a thin film transistor array substrate and a color filter substrate, which are opposite to each other, spacers located to maintain a designated cell gap between the two substrates, and liquid crystals filling the cell gap are provided.
The thin film transistor array substrate includes the gate lines and the data lines, thin film transistors serving as switching elements and formed at the intersections of the gate lines and the data lines, pixel electrodes formed as the unit of liquid crystal cells and connected to the thin film transistors, and an alignment layer applied thereto. The gate lines and the data lines respectively receive signals from the driving circuits through their pad parts.
The thin film transistors respond to a scan signal supplied to the gate lines, and thus supply a pixel voltage signal supplied to the data lines to the pixel electrodes.
The color filter array substrate includes color filters formed as the unit of liquid crystal cells, a black matrix to divide the color filters from each other and reflect external light, a common electrode to supply a reference voltage to the liquid crystal cells in common, and an alignment layer applied thereto.
The thin film transistor array substrate and the color filter array substrate, which are separately manufactured, are aligned, and then are bonded to each other. Thereafter, liquid crystals fill a gap between the two substrates and the gap is sealed, thus completing the display panel.
In the liquid crystal display device, manufactured by the above process, a demand for a touch panel, which recognizes the position of a region touched through a hand or a separate input unit and correspondingly transmits a separate data, has been increased. This touch panel is now used in a state, in which the touch panel is attached to the external surface of a liquid crystal display device. Thus, an attempt to install the touch panel within a panel in the liquid crystal display device has been made.
Now, an example of a liquid crystal display device, in which the above touch panel is installed to prevent the increase in volume caused by the attachment of a separate touch panel to the external surface of the liquid crystal display device, will be described.
Hereinafter, a conventional liquid crystal display device will be described with reference to the accompanying drawings.
FIG. 1 is a schematic circuit diagram illustrating a conventional liquid crystal display device, which recognizes a touch in a capacitance method, and FIG. 2 is a circuit diagram illustrating a capacitance sensor of FIG. 1 and a driving method thereof.
As shown in FIGS. 1 and 2, the conventional liquid crystal display device includes first and second substrates (not shown), which are opposite to each other, a liquid crystal layer (not shown) filling a gap between the first and second substrates, gate lines 11 and data lines 12, which intersect each other on the first substrate to define pixel regions, and thin film transistors (TFTs) formed at the intersections of the gate lines 11 and the data lines 12. A common electrode (not shown, voltage (Vcom)) is formed on the entire surface of the second substrate, and pixel electrodes 13 are respectively formed on the pixel regions on the first substrate.
Here, in order to sense capacitance, a first line 21 located in parallel with the gate lines 11 and a second line 22 located in parallel with the data lines 12 are formed at the outside of the pixel regions, and a first reference voltage line (Vref1) and a second reference voltage line (Vref2) respectively in parallel with the first line 21 and the second line 22 are further formed.
Further, first auxiliary capacitors (Cref1) are formed between the first reference voltage line (Vref1) and the first line 21, and first liquid crystal capacitors (Clc1) are formed between the first line 21 and the common electrode (Vcom). In this case, the first auxiliary capacitors (Cref1) and the first liquid crystal capacitors (Clc1) are formed in series. The first auxiliary capacitors (Cref1) and the first liquid crystal capacitors (Clc1), which are connected in series, are respectively formed in pixels.
In the same manner, second auxiliary capacitors (Cref2) are formed between the second reference voltage line (Vref2) and the second line 22, and second liquid crystal capacitors (Clc2) are formed between the common electrode (Vcom) and the second line 22. The second auxiliary capacitors (Cref2) and the second liquid crystal capacitors (Clc2) are connected in series also.
Here, an amplifier 31 is provided at the end of the first line 21, as shown in FIG. 2, and thus a signal sensed by the first line 21 obtains a value amplified from voltage applied to each of nodes (Vn1) between the respective auxiliary capacitors (Cref) 33 and the respective liquid crystal capacitors (Clc) 32, and whether or not the device is touched and the position of a touched region are determined by the above value. That is, the voltage value of the liquid crystal capacitor (Clc) 32 is varied according to whether or not the device is touched, and in case that the voltage value outputted from the node (Vn1) through the amplifier 31 differs from the initial voltage value of the liquid crystal capacitor (Clc) 32, it is determined that the liquid crystal display device is touched, and correspondingly the position of a touched region is sensed.
Further, first and second switches (sw1 and sw2) are provided at the opposite side to the output side of the node (Vn1) between the auxiliary capacitor (Cref) 33 and the liquid crystal capacitor (Clc) 32, and signals are selectively applied through the first and second switches (sw1 and sw2).
Two common voltage values (Vcomh and VcomI) are alternately applied to the first and second reference voltage lines (Vref1 and Vref2) connected to sides of the first and second auxiliary capacitors (Cref1 and Cref2) 33. Voltage (Va) is applied through the first switch (sw1) and stored in the liquid crystal capacitor (Clc) 32, when the common voltage has the value (Vcom), and then is outputted to the amplifier 31, when the common voltage has the value (VcomI). Consequently, the outputted voltage contains data of the value of the liquid crystal capacitor (Clc) 32, which is varied when touched. A variation in output voltage according to a variation in capacitance is as follows.
            ∂              V                  n          ⁢                                          ⁢          1                            ∂              C        LC              =            -                        C          ref                                      (                                          C                ref                            +                              C                LC                                      )                    2                      ·          (                        V          comH                -                  V          comL                    )      
In this constitution, lines disposed on the X-axis and the Y-axis, which intersect each other, are required, and thus an increase in parasitic capacitance is expected.
The above conventional liquid crystal display device, which recognizes a touch in a capacitance method, has several problems, as follows.
First, a variation in voltage at a point corresponding to one pixel is selectively sensed to detect whether or not the pixel is touched, and thus when several points are touched, it is impossible to recognize whether or not several pixels corresponding to the points are touched.
Second, lines intersecting each other are formed to sense positions of a touched point on the X-axis and the Y-axis to sense a touch, an increase in size of the panel is expected, line resistance of the lines and parasitic capacitance are increased due to the increase in size of the panel, and coupling capacitance is increased and thus a signal to noise (S/N) ratio is decreased. Thereby, the reliability of a signal may be lowered, and thus it may be difficult to recognize the touch.